Rigorous measurement of variables is centrally important to all hypotheses testing research. Measurement presents particular challenges in the behavioral sciences, because of the need to integrate a strong theoretical underpinning with valid and reliable assessment of constructs that are often highly variable and context-dependent. The purpose of the Behavior Measurement Core is to support clinical, prevention, and other cancer research by assisting investigators in identifying or adapting existing measures of key behavioral constructs, or if needed, developing new instruments to fit an investigator research questions and hypotheses. Specifically, the Core supports the following eight services: 1) access to a library of existing behavioral measures relevant for cancer research, 2) expert-guided searches and critical evaluation of concept-driven measures, 3) psychometric evaluation of existing measures, modified measures, and the development of new quantitative measures, 4) assistance with the planning, conduct, collection, and analysis of qualitative data to aid new instrument development, 5) psychometric and statistical analysis of behavioral measures as outcome and predictor variables, 6) behavioral measurement functions in support of project and grant development, 7) integration of Behavioral Measurement Core services with the Biostatistic-Epidemiology and Clinical Trials Cores, and 8) assistance in the preparation of behavioral data for presentation/publication. The availability of behavioral measurement consultations and the database allows investigators to streamline the research process, reduce the turnaround time for project development, and insure inclusion of instruments that are psychometrically sound and well documented in the literature.